[landing] redesign keyserver hero section

Summary:
This diff updates the UI for the top (hero) section of the keyserver page to better match the figma designs. Here is the design for reference:

Depends on D8166

Test Plan:
Please watch the demo video/screenshots:

Desktop/Responsive:

Tablet:

Mobile:

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8167